The present invention relates to a control device for controlling a stepping motor and, in particular, to a device and method for reducing power consumption in electronic watches.
The recent trend is to extend the life of and miniaturize the size of timepieces, such as wristwatches. These objectives can be obtained by reducing the power consumption of the stepping motor used in timepieces which will increase the longevity of the stepping motor and permit the use of a smaller battery thereby conserving space. Also, in recent years, timepieces, such as wristwatches, have been developed in which the battery is replaced with a built-in generator for generating electricity in response to movement of the user's arm. Because it is desirable that these self-generating timepieces be capable of operating continuously for long hours even while left motionless and no electricity is being generated, it is important that power consumption of the stepping motor be minimized.
The stepping motor, also referred to as pulse motor, incremental movement motor or digital motor, is driven by pulse signals and are often used as actuators for digitally controlled devices. Recently, compact-sized electronic devices and information equipment have been developed in which portability is desirable, and compact and light weight stepping motors are in widespread use as actuators for that type of equipment. Representative of such electronic devices are timepieces including electronic watches, time switches and chronographs.
Referring now to FIG. 7, there is shown a timepiece 9, for example a wristwatch, provided with a stepping motor 10, a drive circuit 30 for driving stepping motor 10, a gear train 50 for transferring the force of stepping motor 10 to a second hand 61, a minute hand 62 and an hour hand 63 which are moved by gear train 50. Stepping motor 10 includes a drive coil 11 for producing magnetic force in response to drive pulses output from control device 20, a stator 12 excited by drive coil 11 and a rotor 13 which rotates as a result of the magnetic field excited with stator 12. By selecting a disc-shaped, two-poled permanent magnet for rotor 13, a PM (Permanent Magnet rotation) type stepping motor 10 is formed. Stator 12 is provided with magnetic saturation parts 17 so that opposite magnetic poles that result from the magnetic force generated by drive coil 11 are generated at phases (poles) 15 and 16, respectively, around rotor 13. Also, an internal notching 18 is provided at the appropriate locations on inner periphery of stator 13 so that cogging torque is generated for stopping rotor 13 at appropriate positions.
The rotational energy of rotor 13 is transferred to each of the hands by gear train 50 which includes a fifth wheel 51 which meshes with rotor 13 via a spindle and also meshes with a fourth wheel 52 which meshes with a third wheel 53 which also meshes with a center wheel 54. Center wheel 54 meshes with a minute wheel 55 which meshes with an hour wheel 56. Second hand 61 is mounted on a shaft of fourth wheel 52, minute hand 62 on center wheel 54 and hour hand 63 on hour wheel 56 for displaying time synchronously with the rotation of rotor 13. Of course, a transfer system for displaying the day, month and year (not shown) may also be connected to gear train 50.
In order for timing device 9 to display the time as a result of the rotation of stepping motor 10, stepping motor 10 is supplied with drive pulses which are based on counting signals having a standard frequency (measuring time). Control device 20, which controls stepping motor 10, includes: a pulse synthesizing circuit 22 for generating standard pulses having a standard frequency using a standard oscillation source 21 such as a quartz crystal vibrator, or pulse signals having various pulse widths or timing, and a control circuit 23 for controlling stepping motor 10 based on the various pulses supplied from pulse synthesizing circuit 22. Further, control circuit 23 has a drive control circuit 24 for controlling drive circuit 30 and a detection circuit 25 for detecting whether motor 13 rotated. Drive control circuit 24 includes: a drive pulse supply part 24a for supplying drive pulses to drive circuit 30 which in turn drives rotor 13 of stepping motor 10, a rotation-detecting pulse supply part 24b for producing, after the drive pulses are output, rotation-detecting pulses to induce induction voltage for determining whether rotor 13 has rotated, an auxiliary pulse supply part 24c for producing auxiliary pulses having an effective power that is larger than that of the drive pulses that were output when rotor 13 failed to rotate, a degaussing pulse supply part 24d for producing, after an auxiliary pulse is output, a degaussing pulse having a polarity that is opposite as that of the auxiliary pulse for degaussing driving coil 11 and, a level adjustment part 24e for adjusting the effective power of the driving pulses. Also, detection circuit 25 detects the presence of rotation of rotor 13 by comparing the induced voltage induced by the rotation-detecting pulses with a predetermined value and feeding back the detection to drive control circuit 24.
Drive circuit 30, which supplies drive pulses for driving stepping motor 10 in response to control signals from drive control circuit 24, includes a bridge circuit composed of series-connected p-channel MOSFET 33a and n-channel MOSFET 32b, and p-channel MOSFET 33b and n-channel MOSFET 32a, and is configured to control the voltage to stepping motor 10 from a battery 41. Also, drive circuit 30 includes a pair of resistors 35a and 35b for detecting rotation, each connected in parallel to p-channel MOSFET 33a and 33b, respectively, and p-channel MOSFET 34a and 34b for sampling for supplying chopper pulses to the resistors 35a and 35b. By applying control pulses having various polarities and pulse widths at respective timing from each of the pulse supply parts 24a to 24e of the drive control circuit 24 to each of the gates of MOSFETs 32a, 33a, 33b, 34a and 34b, it is possible to apply to drive coil 11 drive pulses having opposite polarities and pulses for detecting rotation of rotor 13.
Referring now to FIG. 8, there is shown a flow chart of the operation of control device 20. First, in step ST1, standard pulses for measuring time are counted and a one second duration is measured. If it is determined that one second has elapsed, then in step ST2, a drive pulse P1 is produced by drive pulse supply part 24a. Next, in step ST3, a rotation detecting pulse SP2 is produced by rotation detecting pulse supply part 24b for detecting whether rotor 13 has rotated by comparing the induced voltage with a predetermined value in detection circuit 25. If rotation is not detected, rotation of rotor 13 is ensured in step ST4 by supplying an auxiliary pulse P2 from auxiliary pulse supply part 24c to driving coil 11, having effective power that is larger than that of drive pulse P1. After auxiliary pulse P2 is supplied, a degaussing pulse PE is output in step ST5 by degaussing pulse supply part 24d. Next, in step ST5, the effective power of drive pulse P1 is increased by one increment by level adjustment part 24e.
If, in step ST3, the rotation of rotor 13 is detected, then, in step ST7, a counter n is incremented and, in step ST8, counter n is compared to a first predetermined value NO. If counter n is less than first predetermined value NO, operation returns to step ST1. If counter n is equal to first predetermined value NO, which indicates that rotor 13 has rotated consecutively a number of times equal to first predetermined value NO, level adjustment part 24 reduces the effective power of drive pulse P1 by one increment in step S9. Then, in step ST10, counter n is initialized to zero and the next cycle begins.
Referring now to FIG. 9, there is shown a timing chart illustrating the control signals applied to each of gates GP1, GN1 and GS1 of p-channel MOSFET 33a, n-channel MOSFET 32a and p-channel MOSFET 34a, respectively, for inducing a magnetic field having a polarity in one direction in drive coil 11, and the control signals applied to each of gates GP2, GN2 and GS2 of p-channel MOSFET 33b, n-channel MOSFET 32b and p-channel MOSFET 34b, respectively, for inducing a magnetic field having an opposite polarity in drive coil 11. Control device 20 controls the movement of the hands of timepiece 9 once every second by supplying these control signals to drive circuit 30 for controlling stepping motor 10.
First, at time t1, a control signal for producing drive pulse P1, having a pulse width W10, for example, is supplied by drive pulse supply part 24a of drive control circuit 24 to gate GN1 of the n-channel MOSFET 32a and to gate GP1 of the p-channel MOSFET 33a on the driving pole side (i.e. the side of drive circuit 30 from which drive pulse P1 is output). Following the output drive pulse P1, at time t2, a control pulse for producing rotation detecting pulse SP2 for detecting rotation of rotor 13 is supplied by rotation detecting pulse supplying part 24b to gate GP1 of p-channel MOSFET 33a and to gate GS1 of the MOSFET 34a for sampling voltage on the driving pole side. Rotation detection pulse SP2 is a chopping pulse, having duty cycle of about 1/2, and causes the current induced in drive coil 11 when rotor 13 rotates to be output to rotation detecting resistor 35a. The voltage across rotation detecting resistor 35a is compared to a predetermined value in detection circuit 25 to determine whether rotor 13 has rotated. If the voltage induced by rotation detecting pulse SP2 does not reach the predetermined value, it is determined that rotor 13 did not rotate and, in step ST4 at time t3, a control pulse for producing auxiliary pulse P2 is supplied by auxiliary pulse supply part 24c to gate GN1 of the n-channel MOSFET 32a and to gate GP1 of the p-channel MOSFET 33a on the driving pole side. Auxiliary pulse P2, having a pulse width W20 and therefore more effective power larger than drive pulse P1 (where W20&gt;W10), contains sufficient energy to ensure that rotor 13 rotates. After auxiliary pulse P2 is supplied in step ST5 at time t4, a control pulse for outputting degaussing pulse PE is supplied by degaussing pulse supply part 24d to gate GN2 of n-channel MOSFET 32b and to gate GP2 of the p-channel MOSFET 33b on the opposite pole side (reverse pole side). Degaussing pulse PE, which has a polarity that is opposite as that of auxiliary pulse P2, reduces the residual magnetic flux in stator 12 and drive coil 11 that is generated by the large effective power of auxiliary pulse P2. After degaussing pulse PE is supplied and drive pulse P1 is incremented in step ST6, one cycle of operation for diving stepping motor 10 is completed and rotor 13 is successfully rotated by one rotation angle.
At time t11, which is one second after time t1, the next cycle for rotating rotor 13 by one rotation angle begins. In this cycle, MOSFETes 32b, 33b and 34b which were on reverse pole side in the previous cycle are now on the driving pole side. As in the previous cycle, this cycle begins with drive pulse P1 being supplied at time t11. However, if auxiliary pulse P2 was produced in the previous cycle, a drive pulse P1', having effective power raised by one increment by level adjustment part 24e (e.g. drive pulse P1' having a pulse width W11, where W11&gt;W10), is supplied at time t11. Next, at time t12, pulse SP2 is supplied for detecting rotation of rotor 13 and, if no rotation is detected, then, at time t13, auxiliary pulse P2 is output followed by degaussing pulse PE at time 14.
In the next cycle beginning at time t21, a drive pulse P1" having an even wider pulse width, W12 (where W12&gt;W11), is supplied. At time t22, rotation detecting pulse SP2 is output and, if rotation of rotor 13 is detected, which is likely because drive pulse P1" has a high effective power, the cycle ends. After rotor 13 was rotated NO consecutive times in response to drive pulse P1", drive pulse P1" is decremented by level adjustment part 24e and drive pulse P1', having an effective power that is one increment lower than drive pulse P1", is supplied in the next cycle beginning at time t31.
Accordingly, drive pulses having low effective power that is sufficient for continuously driving rotor 13 are supplied so that a small, thin timepiece 9 having accurate hands movements, low power consumption and long life can be provided.
In the conventional system described above, when drive pulse P1 generates insufficient torque to rotate rotor 13 and auxiliary pulse P2 is required to ensure that rotor 13 rotates, the effective power of drive pulse P1 is increased by one increment for the next cycle. However in many cases, a one increment increase in drive pulse P1 also may not be sufficient to rotate rotor 13 and, as a result, a drive pulse increased by two or three increments is supplied. Higher power drive pulses maybe required to drive rotor 13 because of a large increase in meshing load due to a minute variation in positional relationship between the wheel shafts and bearings or variation of meshing positions between wheels as gear train 50 undergoes large torque variations after auxiliary pulse P2 is supplied. Also, because gear train 50, which transmits kinetic energy from stepping motor 10 to the hands, is composed of a plurality of gear wheels, there are times when the meshing load increases periodically due to tolerances in manufacturing or the assembling process of the gear wheels. Once the effective power level of drive pulse P1 is increased by two or three increments and is sufficient to rotate rotor 13, the effective power level of this higher power drive pulse is decremented by one level after a number of rotations of rotor 13, for example, NO rotations, and the effective power of the drive pulse returns to the initial effective power level of drive pulse P1 after an additional consecutive NO rotations. If the meshing load increases at any point in this sequence, the effective power level of the drive pulse is again increased by one or two or even more increments. Therefore, even after rotor 13 has rotated a sufficient amount so that the meshing load condition of gear train 50 is at the level it was before auxiliary pulse P2 was supplied thereby reducing the torque necessary for rotation of rotor 13 to a low level, the effective power of the drive pulse still remains at a somewhat higher level, for example, one or two increments or more, than the minimum required to rotate rotor 13.
In the control method described above, the effective power of drive pulse P1 increases by one increment when the meshing load increases in one angle of rotation of rotor 13 and auxiliary pulse P2 is produced, and the effective power of drive pulse P1 increases by two increments if the meshing load increases during two angles rotation in a given cycle of operation of gear train 50. Furthermore, if the condition of gear train 50 varies due to the torque applied by auxiliary pulse P2, larger torque will still be required to rotate rotor 13 for two or three increments of rotation angle. Accordingly, in the conventional system, even though a control method is used which seeks to supply a drive pulse having the minimum effective power that is sufficient for rotating rotor 13, in many instances the drive pulse that is supplied has an energy level that is several increments higher than the minimum necessary to rotate rotor 13.
Accordingly, it is an object of this invention to provide a control device and method for further reducing the driving power of stepping motor 10 by applying drive pulses having the lowest possible effective power for driving rotor 13 in periods of higher gear train 50 efficiency even though at other periods efficiency worsens due to meshing conditions of gear train 50 caused by auxiliary pulse P2 or manufacturing/assembly tolerances.
A further object of this invention is to provide a control device and method for realizing a small-sized, long-life timepiece having a built in electricity generator that can keep time continuously even after being left motionless for long hours.